1. Field of the Invention
The present invention relates to a differential amplifier, a two-stage amplifier including the differential amplifier, and an analog-to-digital converter including the differential amplifier.
2. Description of the Related Art
With widespread use of digital devices, analog-to-digital converters that convert analog signals into digital signals have been widely used. In connection with high performance and reduction in size and price of digital devices of recent years, it is required for the analog-to-digital converters to have increased resolution, increased processing speed, and reduced power consumption.
The configuration of a basic analog-to-digital converter is described below.
As FIG. 10 shows, an analog-to-digital converter 100 includes a sample-and-hold unit 101 for sampling and holding an analog signal, a reference voltage generator 104 for generating a plurality of different reference voltages by dividing a reference voltage from a reference power supply 102 by using resistors 103 having equal resistances, a comparing unit 106 including a plurality of comparators 105 which each compare the voltage of the analog signal and each reference voltage, and a logic processor 107 for outputting a digital signal corresponding to the analog signal by performing logic processing on the outputs of the comparators 105.
As described above, in the analog-to-digital converter 100, the comparators 105 are connected in parallel to the sample-and-hold unit 101.
As FIG. 11 shows, a chopper comparator 110 including an amplifying circuit 108 whose input end connects to each sampling capacitor 109 is used as each comparator 105.
In other words, in the analog-to-digital converter 100, the sampling capacitors 109 are connected in parallel to the sample-and-hold unit 101 (See, for example, Japanese Unexamined Patent Application Publication No. 11-205144).
Since the analog-to-digital converter 100 of the related art includes the sampling capacitors 109 connected in parallel to the sample-and-hold unit 101, the connected sampling capacitors 109 serve as a load on the sample-and-hold unit 101. This makes it difficult to simultaneously realize increased performance and speed, and reduced power consumption of the analog-to-digital converter 100.
In other words, in order that an analog-to-digital converter using chopper comparators may have increased resolution, the number of chopper comparators must be increased while increasing the accuracy of each chopper comparator. The accuracy of the chopper comparator is dependent on the capacitance of sampling capacitors. Thus, the capacitances of the sampling capacitors in the chopper comparators must be increased. For increasing the accuracy, an increase in the capacitance of the sampling capacitor in each chopper comparator increases the load connected to the sample-and-hold unit. This causes a decrease in the processing speed of the analog-to-digital converter and an increase in the power consumption.
Accordingly, to reduce the power consumption of the analog-to-digital converter, it is possible that, direct supply of the analog signal to a differential amplification circuit prevent the sampling capacitors from being used.
When the analog signal is directly supplied to the differential amplification circuit, there is a possibility that conversion accuracy of the analog-to-digital converter may decrease since noise generated in the differential amplification circuit is superimposed on the analog signal.
Accordingly, it is necessary to provide the input end of the differential amplification circuit with a circuit for preventing the noise from being superimposed. This may cause circuit complexity and large circuit size. In particular, in a device that needs many differential amplifiers, such as the analog-to-digital converter, an increase in circuit size may enlarge the entire device.